Oxidation resistant carbon fiber reinforced carbon composite material and process for producing the same

ABSTRACT

An oxidation resistant carbon fiber reinforced carbon composite material comprises a matrix and 20 volume % or more of carbon fibers, and is characterized in that: the matrix contains ceramic powder that includes boron carbide powder having an average particle diameter of 5 μm or less; and an amount of the ceramic powder is 32 volume % or more based on volume of the carbon fibers.

TECHNICAL FIELD

The present invention relates to a carbon fiber reinforced carboncomposite material having oxidation resistance, and particularly to anoxidation resistant carbon fiber reinforced carbon composite materialused under high-temperature oxidizing atmospheres, which may be used forsliding members for machines, members for metallurgy, trays or shelfboards of heat treat furnaces for sintering ceramics or forheat-treating ferrites, members for hot pressing, materials used inglass-bottle production lines, and furthermore members for aerospaceapplications.

BACKGROUND ART

A carbon fiber reinforced carbon composite material (hereinafterreferred to as a C/C composite) is a material resulting from a greatimprovement in mechanical characteristic disadvantages, such as lowtoughness and brittle fracture, of artificial graphite materials thatare used for electrodes or isotropic graphites. However, the C/Ccomposite is a carbon material, too, and therefore disadvantageouslysuffers from oxidation loss in the air when exposed to a hightemperature of 400 to 500 degrees C. or higher. Thus, the C/C compositeis also limited in its scope of use. As solutions therefor, known areapproaches of: (1) coating the C/C composite with ceramics excellent inoxidation resistance such as SiO₂, B₂O₃—SiC, B₄C, etc.; (2) conversioninto SiC, etc.; (3) combination of (1) and (2) to give the C/C compositea functionally gradient characteristic; (4) impregnating the C/Ccomposite with metal alkoxide to incorporate SiC, B₄C, or oxides thereofinto a whole of the C/C composite; (5) impregnating the C/C compositewith B₂O₃ (Japanese Patent No. 3135129); (6) mixing B₄C into a felt C/Ccomposite during an ordinary production process therefor (JapanesePatent Laid-Open No. H05-306180), and the like.

However, the aforementioned approaches involve the respective problemsthat: (1) it is hard to maintain the oxidation resistance when a coatinglayer is damaged; (2) fibers become brittle, mechanical characteristicsdeteriorate, and vulnerability to thermal shock is increased; (3) costsbecome higher (this approach is unsuitable for general use althoughsuccessful for aerospace applications); (4) the metal alkoxide isexpensive; (5) a heating and impregnating device (device for applyinghot isostatic press) is required and thus costs increase, and, inaddition, the device is limited in size; (6) there can be obtained onlya felt C/C composite of felt type that adopts a felt as a carbon fiberand a pyrolytic carbon as a matrix.

Japanese Patent No. 3058180 discloses a method of applying a mixture ofboron carbide with a thermosetting resin, coal tar, or pitch to serve asa matrix, and a method of incorporating B₄C into a C/C composite bymeans of thermal decomposition of boron-containing gas, to reveal that,at 1200 degrees C., the obtained C/C composite exhibits oxidation lossat a lower speed as compared with materials containing no boron carbide.However, this method fails to propose a definite approach for uniformlydispersing B₄C throughout a C/C composite. In addition, oxidationresistance due to containing of B₄C basically results from anoxidation-resistant protective coating of B₂O₃. At a temperatureexceeding 800 degrees C., accordingly, B₂O₃ evaporates to provide nofundamental oxidation resistance, but nevertheless Japanese Patent No.3058180 shows no evaluation for oxidation at a temperature around 800degrees C.

Japanese Patent Laid-Open No. H04-214073 discloses an oxidationresistant C/C composite that comprises, as a matrix, carbon in whichdispersed are 10 to 50 volume % of ceramic having a particle diameter of10 μm or less (boron carbide, silicon carbide, etc.), and alsocomprises, as a reinforcing material, 20 to 40 volume % of long carbonfibers. However, most Examples therein exhibited weight loss in two tofour hours even at 800 degrees C. in the atmosphere, and thereafterexhibited a tendency of further weight loss. Even the best one of theseExamples exhibited weight loss in seven hours, and thereafter exhibiteda tendency of further weight loss. Accordingly, the material disclosedtherein substantially provides only a small availability. Like this,when considering a matter of costs, the existing circumstances see nooxidation resistant C/C composite available as industrial materials forgeneral industry.

A C/C composite in the most common use today is a 2D-C/C composite usingtwo-dimensional cloths. Therefore, as an oxidation-resistant materialfor general industry as well, the 2D-C/C composite is the most availableand highly required. A production of an oxidation resistant C/Ccomposite adopting the 2D-C/C composite is more difficult than aproduction of the same adopting a felt C/C composite or a 1D-C/Ccomposite using one-dimensional carbon fibers. Thus, there has been aninsufficient development for providing the whole of these materials withoxidation-resistance and self-sealing ability for practical use.

An object of the present invention is to provide an oxidation resistantC/C composite by means of a simple production process, which is widelyavailable for general industry and provided with oxidation resistanceunder a high temperature in the atmosphere, without failing suchcharacteristics of a C/C composite as high strength and high toughness.

DISCLOSURE OF THE INVENTION

Wholehearted investigation by the present inventors in order to solvethe aforementioned problems has revealed considerable importance of aselection and a content of ceramic powder that serves to give sufficientoxidation-resistance to a C/C composite, and also considerableimportance of a method for dispersing the ceramic powder, and thus thepresent invention has been accomplished. An oxidation resistant C/Ccomposite according to the present invention comprises a matrix and 20volume % or more of carbon fibers, and is characterized in that: thematrix contains ceramic powder that includes boron carbide powder havingan average particle diameter of 5 μm or less; and an amount of theceramic powder is 32 volume % or more based on volume of the carbonfibers.

The matrix contains the 32 volume % or more of ceramic powder based onthe volume of the carbon fibers. Therefore, at a high temperature, theceramic powder oxidizes to form an oxidation-resistant protectivecoating throughout the C/C composite. The amount of the ceramic powderis preferably 32 to 76 volume % based on the volume of the carbonfibers. An amount of the ceramic powder less than 32 volume % results ina difficulty in forming the oxidation-resistant protective coatingthroughout the C/C composite. An amount of the ceramic powder more than76 volume % causes an increase in cost and decrease in strength. As anamount of the ceramic powder becomes larger, there can be more tendencythat surplus ceramic powder remains on surfaces of carbon fiber clothsor carbon fiber sheets to make a production of the C/C compositedifficult. However, when the ceramic powder in the matrix is 32 volume %or more based on the volume of the carbon fibers, merely a simple mixingof the ceramic powder during a mixing process causes aggregations of andtherefore local collections of the ceramic powder, thereby failing toform a uniform oxidation-resistant protective coating. In addition,nonuniformity of the C/C composite leads to deterioration incharacteristics such as mechanical strength. This problem can be solvedby sufficiently crushing the ceramic powder with shearing force appliedthereon by using a super mixer, etc., and then mixing the crushedceramic powder such that it may avoid aggregation with another matrix ormatrix precursor as well.

An average particle diameter of the ceramic powder is 5 μm or less,preferably 3 μm or less, and more preferably 1 to 3 μm. By setting theaverage particle diameter of the ceramic powder at 5 μm or less,deterioration in mechanical strength can be prevented and the ceramicpowder can be uniformly dispersed into the matrix. When the averageparticle diameter of the ceramic powder is set at a value much less than1 μm, e.g., 0.5 μm or less, there arises a problem of increased costs.When two or more types of ceramic powder are to be added, it isnecessary that the ceramic powder includes boron carbide powder havingan average particle diameter of 5 μm or less.

As examples of the ceramic powder, there may be mentioned boron carbidepowder in single, or a combination of boron carbide powder with siliconcarbide powder, zirconium boride powder, titanium boride powder,aluminium oxide powder, and the like.

The boron carbide powder and the silicon carbide powder are particularlypreferable as the ceramic powder. As for the type of the ceramic powder,a single use of the boron carbide can prevent oxidation loss in atemperature range substantially up to 800 degrees C. A single use of thesilicon carbide results in a difficulty in forming anoxidation-resistant coating, and thus oxidation resistance undergoessubstantially no improvement. In this case, therefore, availability asindustrial materials is small. In case of a mixture of two types ofpowder, i.e., the boron carbide powder and the silicon carbide powder,oxidation resistance can be maintained at a temperature exceeding 800degrees C. An oxidation resistance temperature is determined dependingon viscosity and an evaporating temperature of an oxidation-resistantprotective coating that is formed at a high temperature in an oxidizingatmosphere. The oxidation resistance temperature is controlled inaccordance with combinations of selected ceramic powders. When the boroncarbide powder is singularly used, for example, formed is anoxidation-resistant coating comprising glassy boron oxide. When theboron carbide powder and the silicon carbide powder are used, a glasslayer comprising mixed boron oxide and silicon oxide is formed to serveas an oxidation-resistant coating. In this way, conditions could be setin accordance with purposes such that there may be formed anoxidation-resistant coating capable of developing oxidation resistancewithin a significant temperature range. For example, in order to developoxidation resistance over as wide a temperature range as possible, it ispreferable that an amount of boron carbide mixed with silicon carbideis, by weight, approximately 1.2 times amount of the silicon carbide.

A volume fraction of the carbon fibers is 20 volume % or more, andpreferably 20 to 50 volume %. When a volume fraction of the carbonfibers exceeds 50 volume %, binding power between carbon fiber cloths orcarbon fiber sheets cannot sufficiently be maintained, and therefore aproduction of a C/C composite becomes practically difficult in thepresent invention. When a volume fraction of the carbon fibers is lessthan 20 volume %, there are small effects of the carbon fibers, withouthigh toughness and mechanical strength.

The carbon fibers comprise any one of a fine-woven spun-yarn cloth; atwo-dimensional-woven cloth of continuous threads of long carbon fibers;a one-dimensional long carbon fiber sheet; short carbon fibers and afine-woven spun-yarn cloth; short carbon fibers and atwo-dimensional-woven cloth of continuous threads of long carbon fibers;and short carbon fibers and a one-dimensional long carbon fiber sheet.It is also possible to use a combination of any two or more among thefine-woven spun-yarn cloth, the cloth of continuous threads of longcarbon fibers, and the one-dimensional long carbon fiber sheet. Inparticular, a combination of the fine-woven spun-yarn cloth and thecloth of continuous threads of long carbon fibers results in a C/Ccomposite having high toughness.

As the two-dimensional-woven cloth (hereinafter referred to as a 2Dcloth), there may be used ones in which a fiber diameter is 10 to 20 μmand the number of filaments is 1000 to 12000. In addition, such a clothmay be selected, in accordance with its intended use, from satin-wovenor plain-woven cloths of continuous threads of long fibers, and afine-woven spun-yarn cloth obtained by spinning fibers, etc.

The matrix includes a carbon compound resulting from liquid syntheticresin or from liquid synthetic resin and powdery synthetic resin.Moreover, the matrix includes mesocarbon microbeads and/or carbon powderhaving 5 to 15 weight % of residual volatile matter.

A phenolic resin capable of high yield of carbonization is preferablyadopted as the synthetic resin. Production costs can thereby be reduced.In addition, an adoption of the phenolic resin allows short carbon fiberchops to be added as a reinforcer. Instead of the phenolic resin, afuran resin, etc., may also be adopted. Ethyl alcohol, etc., may beadopted as a solvent.

The oxidation resistant carbon fiber reinforced carbon compositematerial according to the present invention comprises the matrix and 20volume % or more of carbon fibers. A slurried matrix precursor is formedby mixing the liquid synthetic resin with the ceramic powder thatincludes, based on the volume of the carbon fibers, 32 volume % or moreof boron carbide powder having an average particle diameter of 5 μm orless. The resulting matrix precursor is then uniformly applied tocloth-form or sheet-form carbon fibers, which are subsequently put inlayers and baked, to thereby obtain the present oxidation resistantcarbon fiber reinforced carbon composite material.

Alternatively, the slurried matrix precursor is uniformly infiltratedinto cloth-form or sheet-form carbon fibers and, further, uniformlyapplied to surfaces of the carbon fibers, which are subsequently put inlayers and baked. The matrix precursor can thereby be spread throughoutthe carbon fibers.

Alternatively, the slurried matrix precursor is uniformly infiltratedinto cloth-form or sheet-form carbon fibers and, further, uniformlyapplied to surfaces of the carbon fibers, which are subsequently put inlayers, baked, and then further impregnated with pitch or syntheticresin to be carbonized.

In these cases, a viscosity of the slurried matrix precursor is adjustedat 5 to 40 mPa-s, and preferably at 8 to 25 mPa-s. When this viscosityis high, infiltration of the matrix precursor into the carbon fibercloths or carbon fiber sheets becomes insufficient. When the viscosityis too low, on the other hand, it becomes hard to have a sufficientamount of boron carbide powder be present on a unit area.

The matrix precursor may also contain the mesocarbon microbeads, thecarbon powder including 5 to 15 weight % of residual volatile matter,the short carbon fibers, and the like. This leads to improvements inmechanical characteristics.

It is also possible that ceramic powder and any one or more amongsynthetic resin powder, mesocarbon microbeads, carbon powder including 5to 15 weight % of residual volatile matter, and short carbon fibers, areintegrally made into solid fine particles to obtain powder, and then aslurried matrix precursor mixed with the obtained powder is uniformlyapplied to cloth-form or sheet-form carbon fibers, which aresubsequently put in layers and baked. Moreover, it is also possible thata slurried matrix precursor is uniformly applied to cloth-form orsheet-form carbon fibers, and then powder of solid fine particlesintegrally made from ceramic powder and any one or more among syntheticresin powder, mesocarbon microbeads, carbon powder including 5 to 15weight % of residual volatile matter, and short carbon fibers, isuniformly applied further to surfaces of the cloth-form or sheet-formcarbon fibers, which are subsequently put in layers and baked.

Thereby, each of the synthetic resin powder, the mesocarbon microbeads,the carbon powder including 5 to 15 weight % of residual volatilematter, the short carbon fibers, and the ceramic powder can disperseuniformly, and at the same time a binder can effectively demonstrate itsfunctions, so as to improve mechanical characteristics. For an operationof integrally making the solid fine particles, used is, for example, acommercially available apparatus for compositing fine particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b), and 1(c) are tables showing particulars of Examplesand Comparative Examples of the present invention;

FIG. 2 is a graph showing weight loss due to oxidation at 800 degrees C.in the atmosphere with respect to Examples 1, 4, 7, 13 and 16, andComparative Examples 1, 5 and 6 of the present invention;

FIG. 3 is a graph showing weight loss due to oxidation at 900 degrees C.in the atmosphere with respect to Examples 1, 10, 13 and 16, andComparative Examples 4, 5 and 6 of the present invention;

FIG. 4 is a graph showing weight loss due to oxidation at 1000 degreesC. in the atmosphere with respect to Examples 10 and 16, and ComparativeExample 4 of the present invention;

FIG. 5 is a graph showing weight loss due to oxidation at 1200 degreesC. in the atmosphere with respect to Examples 10 and 16, and ComparativeExample 4 of the present invention; and

FIG. 6 is a graph showing weight loss due to oxidation at 800 degrees C.in the atmosphere with respect to Examples 19, 20, 21 and 22, andComparative Example 7 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

For an oxidation resistant C/C composite of the present invention,firstly, carbon fibers and a matrix precursor are combined to prepare apreform. During a preparation of the preform, it is necessary to combinethe matrix and the carbon fibers in a sufficiently uniform manner.Configurations of the carbon fibers include two-dimensional cloths,one-dimensional sheets, and the like. A slurried matrix precursor isapplied to these carbon fibers. This application of the slurried matrixprecursor is performed by applying the slurry to surfaces of the carbonfibers using brushes or doctor blades, or by rubbing and infiltratingthe slurry into the carbon fibers. Here, a viscosity of the slurriedmatrix precursor is adjusted at 5 to 40 mPa-s, and preferably at 8 to 25mPa-s. When this viscosity is high, infiltration of the matrix precursorinto the carbon fiber cloths or carbon fiber sheets becomesinsufficient. When the viscosity is too low, on the other hand, itbecomes hard to have a sufficient amount of boron carbide powder bepresent on a unit area. The viscosity can be adjusted by properly addingethyl alcohol, etc., and the like.

For the matrix precursor, a synthetic resin as a binder component ismixed with mesocarbon microbeads, carbon powder including volatilematter, or short carbon fibers as other components. Although noparticular limitation is put on a type of the synthetic resin, phenolicresins and furan resins are suitable because of their high yield ofcarbonization and easy availability in terms of costs, etc. In case ofusing and mixing a synthetic resin taking a liquid form in the ordinarytemperature, firstly ceramic powder is dispersed into alcohols such asethanol in order to avoid reaggregation, and subsequently the obtaineddispersion liquid is stirred well together with the synthetic resin andother matrix components into a slurry form, to thereby form the matrixprecursor.

In case of using a synthetic resin taking a solid powdery form in theordinary temperature and mixing the synthetic resin powder withmesocarbon microbeads, carbon powder including volatile matter, shortcarbon fiber chops and the like; these powders and the ceramic powderare made into solid fine particles so that they may integrate with eachother. Thereby, each of these powders can disperse uniformly, and at thesame time a binder can effectively demonstrate its functions, so as toimprove mechanical characteristics. For an operation of making the solidfine particles, used is, for example, a commercially available apparatusfor compositing fine particles.

A combined use of the above-mentioned slurry and solid fine particles isalso acceptable. That is, for example, the slurry is applied to carbonfiber cloths or carbon fiber sheets, on which the solid fine particlesmay subsequently be put, and then carbon fiber cloths or carbon fibersheets may further be laminated thereon.

Other than the synthetic resin, for example, mesocarbon microbeadshaving approximately 10 weight % of volatile matter, or carbon powderhaving 5 to 15 weight %, preferably 7 to 12 weight %, of volatilematter, may be added as a component of the matrix precursor fordensification of the matrix precursor. In this case, when an amount ofcarbon to be generated from these components is set at a large value,large cracks are formed in the matrix and therefore mechanical strengthconsiderably deteriorates. This is because the mesocarbon microbeads andthe carbon powder having volatile matter incur contractions while thecarbon fibers incur no contraction, so that internal stress arises. Inrelieving this internal stress, the above-mentioned cracks are formed.In order to prevent the cracks, it is necessary that mesocarbonmicrobeads and carbon powder having as high volatile matter as more than15 weight % are not used, and that an amount of carbon resulting frommesocarbon microbeads and carbon powder having volatile matter is set atapproximately 30 volume % or less of a total matrix. The remainingproportion is occupied by the ceramic powder and resin carbon resultingfrom the synthetic resin.

The cloths or sheets applied with the slurry are put in layers having apredetermined thickness, e.g., a thickness of 3 to 30 mm, to form aprepreg layered structure. The prepreg layered structure is hot-pressedand formed into a preform. Conditions in the hot-pressing should bedetermined in accordance with a type of synthetic resin used, a type andamount of carbon fibers used, and a size of the preform, etc. Generally,however, the hot press is performed under a pressure of 5 to 15 kg/cm²at a maximum temperature of 150 to 300 degrees C. The obtained preformis carbonized and thereby made into an oxidation resistant C/Ccomposite. In this process, the preform is subjected to heat treatmentof 800 to 2000 degrees C. in a non-oxidizing atmosphere such as anitrogen gas atmosphere. Variation in heat-treatment temperature causesno prominent change in oxidation resistance and in mechanicalcharacteristics, as long as the variation stays within theabove-mentioned temperature range. Accordingly, the heat treatment couldbe performed at a temperature suitable for applications of a resultingC/C composite.

In addition, a temperature up to which oxidation resistance ismaintained can be controlled by selecting a type and a combination ofthe ceramic powder used. For example, when an operation temperaturerange of the C/C composite is 800 degrees C. or less in the atmosphere,boron carbide powder is adopted as the ceramic powder. In a rangeexceeding 800 degrees C. up to 1200 degrees C., both of boron carbidepowder and silicon carbide powder are used, to thereby obtain anoxidation resistant C/C composite that suffers from substantially alittle oxidation loss. Like this, the oxidation resistant C/C compositeof the present invention can be formed through a simple process, havesufficient toughness, and maintain good points of the mechanicalcharacteristics of a C/C composite.

After the heat treatment process, the preform may further be impregnatedwith pitch or a synthetic resin, for its carbonization. Even after thisprocess, oxidation resistance characteristics do not deteriorate.Therefore, oxidation resistance is improved, and at the same timefurther densification is achieved due to impregnation with the pitch orthe synthetic resin, so that the mechanical characteristics can also beimproved.

In the following, the present invention will specifically be describedthrough Examples.

EXAMPLE 1

Boron carbide powder having an average particle diameter of 3 μm wasadded to ethyl alcohol such that a volume fraction of the boron carbidepowder in a matrix may be 51 volume %, and then the boron carbide powderwas well stirred and dispersed. An amount of the ethyl alcohol used wassubstantially equal to an amount of synthetic resin to be added in thenext step. A resol phenolic resin as a liquid synthetic resin was addedthereto at a weight ratio of 1.1 with respect to carbon fibers, and thenwell stirred and dispersed to be made into slurry, so that a matrixprecursor was prepared.

The slurry was uniformly applied to 2D fine-woven cloths (which arePAN-based ones having a fiber diameter of about 10 μm) using a doctorblade. At this time, in order to infiltrate the slurry into the cloths,this process involved an ingenuity of applying in advance a part of theslurry to the cloths in a rubbing manner and then applying the remainingslurry to surfaces of the cloths. The cloths having the slurry appliedthereto were put in layers and air-dried. The cloth had a size of 80mm×80 mm, and fifteen cloths were put in layers to form a prepreglayered structure having a thickness of about 7 mm.

Next, a thermal pressure molding was conducted in accordance with aconventional method. In this molding, a pressure of about 10 kg/cm² wasapplied to the prepreg layered structure, and, in this condition, apressure application started from 110 degrees C. Then, the prepreglayered structure was maintained at 160 degrees C. for an hour.Subsequently, the prepreg layered structure was subjected to a heattreatment at 260 degrees C. for 16 hours using a dryer, so that apreform was formed.

The obtained preform was baked under a heat treatment in a stream ofnitrogen gas at a temperature rise rate of about 10 degrees C./hour upto 1000 degrees C. Thereby, a C/C composite was obtained.

EXAMPLE 2

A C/C composite was obtained through the same procedure as in Example 1,except that boron carbide powder having an average particle diameter of1 μm was adopted.

EXAMPLE 3

A C/C composite was obtained through the same procedure as in Example 1,except that boron carbide powder having an average particle diameter of1 μm was added such that its volume fraction in a matrix may be 56volume %, and such that a resol phenolic resin was added at a weightratio of 0.9 with respect to carbon fibers.

EXAMPLE 4

As a matrix precursor, boron carbide powder having an average particlediameter of 1 μm was sufficiently mixed with carbon powder with shearingforce applied thereon by using a super mixer. An amount of the boroncarbide powder was such that its volume fraction in a matrix may be 52volume %. An amount of the carbon powder was such that its weight ratioto carbon fibers may be 0.4. The mixer rotated its blades at a speed of2000 rpm for three minutes. A lack of this process causes a difficultyin sufficient mixing of the boron carbide powder and the carbon powders.Resulting mixed powder was well stirred in and dispersed into ethylalcohol. An amount of the alcohol used was substantially equal to anamount of resin to be added in the next step. Next, a resol phenolicresin was added thereto at a weight ratio of 1.1 with respect to carbonfibers, and then well stirred and dispersed to be made into slurry. AC/C composite was obtained through the same procedure as in Example 1except for the above.

EXAMPLE 5

As a matrix precursor, boron carbide powder having an average particlediameter of 1 μm was sufficiently mixed with mesocarbon microbeads withshearing force applied thereon by using a super mixer. An amount of theboron carbide powder was such that its volume fraction in a matrix maybe 52 volume %. An amount of the mesocarbon microbeads was such that itsweight ratio to carbon fibers may be 0.4. The mixer rotated its bladesat a speed of 2000 rpm for three minutes. A lack of this process causesa difficulty in sufficient mixing of the boron carbide powder and themesocarbon microbeads. Resulting mixed powder was well stirred in anddispersed into ethyl alcohol. An amount of the alcohol used wassubstantially equal to an amount of resin to be added in the next step.Next, a resol phenolic resin was added thereto at a weight ratio of 1.1with respect to carbon fibers, and then well stirred and dispersed to bemade into slurry. A C/C composite was obtained through the sameprocedure as in Example 1 except for the above.

EXAMPLE 6

For a matrix precursor, two types of matrix precursor were prepared. Asfor one of them, boron carbide powder having an average particlediameter of 1 μm was well stirred in and dispersed into ethyl alcohol.An amount of the boron carbide powder was such that its volume fractionin a matrix may be 26 volume %. A resol phenolic resin was added theretoat a weight ratio of 1.1 with respect to carbon fibers, and then wellstirred and dispersed to be made into slurry (which is referred to as“A”). As for the other matrix precursor, boron carbide powder having anaverage particle diameter of 1 μm, carbon powder, and powdery phenolicresin as synthetic resin powder were sufficiently mixed with one anotherwith shearing force applied thereon by using a super mixer. An amount ofthe boron carbide powder was such that its volume fraction in a matrixmay be 26 volume %. An amount of the carbon powder was such that itsweight ratio to carbon fibers may be 0.4. An amount of the powderyphenolic resin was such that its weight ratio to carbon fibers may be0.5. The mixer rotated its blades at a speed of 1500 rpm for threeminutes. A resulting mixture was made into composite fine particlesusing a hybridizer of Nara Machinery Co., Ltd., for three minutes at ablade rotational speed of 12000 rpm (an obtained material is referred toas “B”).

Then, the matrix precursor A was mixed with the matrix precursor B (inpowdery form), and their mixture was uniformly applied to fine-wovencloths in the manner described in Example 1. Fifteen of these clothswere put in layers to form a prepreg layered structure having athickness of about 9 mm.

A molding was performed in which a pressure of about 10 kg/cm² wasapplied to the prepreg layered structure, and, in this condition, apressure application started from 110 degrees C. Then, the prepreglayered structure was maintained at 260 degrees C. for an hour.Subsequently, the prepreg layered structure was subjected to a heattreatment at 300 degrees C. for 16 hours in a dryer to form a preform.Thereafter, the same procedure as in Example 1 was performed, to obtaina C/C composite.

EXAMPLE 7

For a matrix precursor, boron carbide powder having an average particlediameter of 3 μm was sufficiently mixed with short carbon fibers havingan average length of 30 μm with shearing force applied thereon by usinga super mixer. An amount of the boron carbide powder was such that itsvolume fraction in a matrix may be 51 volume %. An amount of the shortcarbon fibers was such that their volume fraction in total materials maybe 10 volume %. The mixer rotated its blades at a speed of 2000 rpm forthree minutes. Resulting mixed powder was well stirred in and dispersedinto ethyl alcohol. An amount of the alcohol used was substantiallyequal to an amount of resin to be added in the next step. Next, a resolphenolic resin was added thereto at a weight ratio of 1.1 with respectto carbon fibers contained in materials, and then well stirred anddispersed to be made into slurry. A C/C composite was obtained throughthe same procedure as in Example 1 except for the above.

EXAMPLE 8

For a matrix precursor, boron carbide powder having an average particlediameter of 3 μm was sufficiently mixed with short carbon fibers havingan average length of 1000 μm with shearing force applied thereon byusing a super mixer. An amount of the boron carbide powder was such thatits volume fraction in a matrix may be 51 volume %. An amount of theshort carbon fibers was such that their volume fraction in totalmaterials may be 5 volume %. The mixer rotated its blades at a speed of2000 rpm for three minutes. Resulting mixed powder was well stirred inand dispersed into ethyl alcohol. An amount of the alcohol used wassubstantially equal to an amount of resin to be added in the next step.Next, a resol phenolic resin was added thereto at a weight ratio of 1.1with respect to carbon fibers in materials, and then well stirred anddispersed to be made into slurry. A C/C composite was obtained throughthe same procedure as in Example 1 except for the above.

EXAMPLE 9

For a matrix precursor, boron carbide powder having an average particlediameter of 3 μm and silicon carbide powder having an average particlediameter of 3 μm were well stirred in and dispersed into ethyl alcoholsuch that a volume fraction of ceramic powder in a matrix may be 51volume %. A volume fraction of the boron carbide powder to carbon fibersin materials was 57 volume %. A volume fraction of the silicon carbidepowder to the carbon fibers in the materials was 38 volume %. An amountof the alcohol used was substantially equal to an amount of resin to beadded in the next step. Next, a resol phenolic resin was added theretoat a weight ratio of 1.5 with respect to carbon fibers in materials, andthen well stirred and dispersed to be made into slurry. A C/C compositewas obtained through the same procedure as in Example 1 except for theabove.

EXAMPLE 10

For a matrix precursor, two matrix precursors were prepared. As for onematrix precursor, boron carbide powder having an average particlediameter of 3 μm and silicon carbide powder having an average particlediameter of 3 μm were well stirred in and dispersed into ethyl alcohol.A volume fraction of the boron carbide powder to carbon fibers inmaterials was 29 volume %. A volume fraction of the silicon carbidepowder to the carbon fibers in the materials was 19.5 volume %. Anamount of the alcohol used was substantially equal to an amount of resinto be added in the next step. Next, a resol phenolic resin was addedthereto at a weight ratio of 0.8 with respect to carbon fibers inmaterials, and then well stirred and dispersed to be made into slurry(which is referred to as “A”). As for the other matrix precursor, boroncarbide powder having an average particle diameter of 3 μm, siliconcarbide powder having an average particle diameter of 3 μm, short carbonfibers having an average length of 30 μm, and a powdery phenolic resinwere sufficiently mixed with one another with shearing force appliedthereon by using a super mixer. A volume fraction of the boron carbidepowder to carbon fibers in materials was 29 volume %. A volume fractionof the silicon carbide powder to the carbon fibers in the materials was19.5 volume %. A volume fraction of the short carbon fibers to totalmaterials was 7 volume %. A weight ratio of the powdery phenolic resinto the carbon fibers in the materials was 0.4. The mixer rotated itsblades at a speed of 1500 rpm for three minutes. A resulting mixture wasmade into composite fine particles using a hybridizer of Nara MachineryCo., Ltd., for three minutes at a blade rotational speed of 12000 rpm(an obtained material is referred to as “B”). A C/C composite wasobtained through the same procedure as in Example 6 except for theabove.

EXAMPLE 11

During a preparation of the matrix B of Example 10, short carbon fibershaving an average length 1000 μm were used. A volume fraction of theshort carbon fibers to total materials was 5 volume %. A C/C compositewas obtained through the same procedure as in Example 10 except for theabove.

EXAMPLE 12

For a matrix precursor, two matrix precursors were prepared. As for onematrix precursor, boron carbide powder having an average particlediameter of 3 μm and silicon carbide powder having an average particlediameter of 10 μm were well stirred in and dispersed into ethyl alcohol.A volume fraction of the boron carbide powder to carbon fibers inmaterials was 28.5 volume %. A volume fraction of the silicon carbidepowder to the carbon fibers in the materials was 19 volume %. An amountof the alcohol used was substantially equal to an amount of resin to beadded in the next step. Next, a resol phenolic resin was added theretoat a weight ratio of 0.8 with respect to the carbon fibers in thematerials, and then well stirred and dispersed to be made into slurry(which is referred to as “A”). As for the other matrix precursor, boroncarbide powder having an average particle diameter of 3 μm, siliconcarbide powder having an average particle diameter of 10 μm, and apowdery phenolic resin were sufficiently mixed with one another withshearing force applied thereon by using a super mixer. A volume fractionof the boron carbide powder to the carbon fibers in the materials was28.5 volume %. A volume fraction of the silicon carbide powder to thecarbon fibers in the materials was 19 volume %. A weight ratio of thepowdery phenolic resin to the carbon fibers in the materials was 0.4.The mixer rotated its blades at a speed of 1500 rpm for three minutes. Aresulting mixture was made into composite fine particles using ahybridizer of Nara Machinery Co., Ltd., for three minutes at a bladerotational speed of 12000 rpm (an obtained material is referred to as“B”). A C/C composite was obtained through the same procedure as inExample 6 except for the above.

EXAMPLE 13

A C/C composite was obtained through the same procedure as in Example 1,except that plain-woven cloths were adopted as carbon fibers, that anamount of boron carbide powder having an average particle diameter of 3μm was such that its volume fraction in a matrix may be 59 volume %, andthat a resol phenolic resin was added at a weight ratio of 1.25 withrespect to carbon fibers.

EXAMPLE 14

A C/C composite was obtained through the same procedure as in Example 1,except that plain-woven cloths were adopted as carbon fibers, that anamount of boron carbide powder having an average particle diameter of 1μm was such that its volume fraction in a matrix may be 51 volume %, andthat a resol phenolic resin was added at a weight ratio of 1.1 withrespect to carbon fibers.

EXAMPLE 15

For a matrix precursor, two matrix precursors were prepared. As for onematrix precursor, boron carbide powder having an average particlediameter of 3 μm was well stirred in and dispersed into ethyl alcohol.An amount of the boron carbide powder was such that its volume fractionin a matrix may be 28.5 volume %. An amount of the alcohol used wassubstantially equal to an amount of resin to be added in the next step.Next, a resol phenolic resin was added thereto at a weight ratio of 0.8with respect to carbon fibers in materials, and then well stirred anddispersed to be made into slurry (which is referred to as “A”). As forthe other matrix precursor, boron carbide powder having an averageparticle diameter of 3 μm and a powdery phenolic resin were sufficientlymixed with each other with shearing force applied thereon by using asuper mixer. A volume fraction of the boron carbide in a matrix was 28.5 volume %. A weight ratio of the powdery phenolic resin to the carbonfibers in the materials was 0.4. The mixer rotated its blades at a speedof 1500 rpm for three minutes. A resulting mixture was made intocomposite fine particles using a hybridizer of Nara Machinery Co., Ltd.,for three minutes at a blade rotational speed of 12000 rpm (an obtainedmaterial is referred to as “B”).

As the carbon fibers, adopted were plain-woven cloths. A C/C compositewas obtained through the same procedure as in Example 6 except for theabove.

EXAMPLE 16

For a matrix precursor, two matrix precursors were prepared. As for onematrix precursor, boron carbide powder having an average particlediameter of 1 μm and silicon carbide powder having an average particlediameter of 3 μm were well stirred in and dispersed into ethyl alcohol.A volume fraction of the boron carbide powder to carbon fibers inmaterials was 30 volume %. A volume fraction of the silicon carbidepowder to the carbon fibers in the materials was 20 volume %. An amountof the alcohol used was substantially equal to an amount of resin to beadded in the next step. Next, a resol phenolic resin was added theretoat a weight ratio of 0.8 with respect to the carbon fibers in thematerials, and then well stirred and dispersed to be made into slurry(which is referred to as “A”). As for the other matrix precursor, boroncarbide powder having an average particle diameter of 1 μm, siliconcarbide powder having an average particle diameter of 3 μm, and apowdery phenolic resin were sufficiently mixed with one another withshearing force applied thereon by using a super mixer. A volume fractionof the boron carbide powder to the carbon fibers in the materials was 30volume %. A weight ratio of the silicon carbide powder to the carbonfibers in the materials was 0.4. A volume fraction of the powderyphenolic resin to the carbon fibers in the materials was 40 volume %.The mixer rotated its blades at a speed of 1500 rpm for three minutes. Aresulting mixture was made into composite fine particles using ahybridizer of Nara Machinery Co., Ltd., for three minutes at a bladerotational speed of 12000 rpm (an obtained material is referred to as“B”). A C/C composite was obtained through the same procedure as inExample 6 except for the above.

EXAMPLE 17

A C/C composite was obtained through the same procedure as in Example 1,except for an adoption of one-dimensional sheets as carbon fibers.

EXAMPLE 18

For a matrix precursor, boron carbide powder having an average particlediameter of 1 μm and silicon carbide powder having an average particlediameter of 3 μm were well stirred in and dispersed into ethyl alcoholsuch that a volume fraction of ceramic powder in a matrix may be 59volume %. A volume fraction of the boron carbide powder to carbon fibersin materials was 60 volume %. A volume fraction of the silicon carbidepowder to the carbon fibers in the materials was 40 volume %. An amountof the alcohol used was substantially equal to an amount of resin to beadded in the next step. Next, a resol phenolic resin was added theretoat a weight ratio of 1.1 with respect to the carbon fibers in thematerials, and then well stirred and dispersed to be made into slurry.As the carbon fibers, adopted were one-dimensional-sheets. A C/Ccomposite was obtained through the same procedure as in Example 1 exceptfor the above.

EXAMPLE 19

Boron carbide having an average particle diameter of 3 μm was added intoethyl alcohol, and well stirred and dispersed. An amount of the boroncarbide was 43 parts by volume based on 100 parts by volume of carbonfibers. The boron carbide powder was in advance sufficiently mixed andcrushed with shearing force applied thereon by using a super mixer. Thesuper mixer performed mixing for three minutes at a blade rotationalspeed of 1500 rpm. The ethyl alcohol was 1.7 times the boron carbide byweight. A resol phenolic resin as a liquid synthetic resin was addedthereto at a weight ratio of 0.8 with respect to carbon fibers, and thenwell stirred and dispersed to be made into slurry, so that a matrixprecursor was prepared. The slurry was uniformly applied to 2Dfine-woven spun-yarn cloths (which are PAN-based ones having a fiberdiameter of about 10 μm) using a doctor blade. At this time, in order toinfiltrate the slurry into the cloths, this process involved aningenuity of applying in advance a part of the slurry to the cloths in arubbing manner and then applying the remaining slurry to surfaces of thecloths. Here, a viscosity of the slurry at a working temperature was 11mPa-s. The cloths having the slurry applied thereto were put in layersand air-dried. The cloth had a size of 80 mm×80 mm, and fifteen clothswere put in layers to form a prepreg layered structure having athickness of about 5 mm. Next, a thermal pressure molding was conductedin accordance with a conventional method. In this molding, a pressure ofabout 40 kg/cm² was applied to the prepreg layered structure, and, inthis condition, a pressure application started from 110 degrees C. Then,the prepreg layered structure was maintained at 160 degrees C. for anhour. Subsequently, the prepreg layered structure was subjected to aheat treatment at 260 degrees C. for 16 hours using a dryer, to form apreform.

The preform was baked through a heat treatment in a stream of nitrogengas at a temperature rise rate of about 10 degrees C./hour up to 1000degrees C. Thereby, a C/C composite was obtained. Further, under avacuum pressure, the obtained C/C composite was impregnated with aliquid phenolic resin. Then, an impregnated C/C composite was baked andcarbonized through a heat treatment at 1000 degrees C. in a stream ofnitrogen gas. An obtained material had a bulk density of 1.63 g/cm³ anda bending strength of 90 MPa.

EXAMPLE 20

Boron carbide having an average particle diameter of 3 μm was added intoethyl alcohol, and well stirred and dispersed. An amount of the boroncarbide was 43 parts by volume based on 100 parts by volume of carbonfibers. The boron carbide powder was in advance sufficiently mixed andcrushed with shearing force applied thereon by using a super mixer. Thesuper mixer performed mixing for three minutes at a blade rotationalspeed of 1500 rpm. The ethyl alcohol was 1.7 times the boron carbide byweight. A resol phenolic resin as a liquid synthetic resin was addedthereto at a weight ratio of 1.2 with respect to carbon fibers, and thenwell stirred and dispersed to be made into slurry, so that a matrixprecursor was prepared. The slurry was uniformly applied to fine-wovenspun-yarn cloths of 2D cloths (which are PAN-based ones having a fiberdiameter of about 10 μm) using a doctor blade. At this time, in order toinfiltrate the slurry into the cloths, this process involved aningenuity of applying in advance a part of the slurry to the cloths in arubbing manner and then applying the remaining slurry to surfaces of thecloths. Here, a viscosity of the slurry at a working temperature was 13mPa-s. The cloths having the slurry applied thereto were put in layersand air-dried. The cloth had a size of 80 mm×80 mm, and fifteen clothswere put in layers to form a prepreg layered structure having athickness of about 5 mm. Next, a thermal pressure molding was conductedin accordance with a conventional method. In this molding, a pressure ofabout 40 kg/cm² was applied to the prepreg layered structure, and, inthis condition, a pressure application started from 110 degrees C. Then,the prepreg layered structure was maintained at 160 degrees C. for anhour. Subsequently, the prepreg layered structure was subjected to aheat treatment at 260 degrees C. for 16 hours using a dryer, to form apreform.

The preform was baked through a heat treatment in a stream of nitrogengas at a temperature rise rate of about 10 degrees C./hour up to 1000degrees C. Thereby, a C/C composite was obtained. An obtained materialhad a bulk density of 1.33 g/cm³ and a bending strength of 34 MPa.

EXAMPLE 21

Boron carbide having an average particle diameter of 3 μm was added intoethyl alcohol, and well stirred and dispersed. An amount of the boroncarbide was 76 parts by volume based on 100 parts by volume of carbonfibers. The boron carbide powder was in advance sufficiently mixed andcrushed with shearing force applied thereon by using a super mixer. Thesuper mixer performed mixing for three minutes at a blade rotationalspeed of 1500 rpm. A weight of the ethyl alcohol used was equal to thatof the boron carbide. A resol phenolic resin as a liquid synthetic resinwas added thereto at a weight ratio of 1.2 with respect to carbonfibers, and then well stirred and dispersed to be made into slurry, sothat a matrix precursor was prepared. The slurry was uniformly appliedto 3K plain-woven cloth of 2D cloth (which are PAN-based ones having afiber diameter of about 10 μm) using a doctor blade. At this time, inorder to infiltrate the slurry into the cloths, this process involved aningenuity of applying in advance a part of the slurry to the cloths in arubbing manner and then applying the remaining slurry to surfaces of thecloths. Here, a viscosity of the slurry at a working temperature was 23mPa-s. The cloths having the slurry applied thereto were put in layersand air-dried. The cloth had a size of 80 mm×80 mm, and fifteen clothswere put in layers to form a prepreg layered structure having athickness of about 4 mm. Next, a thermal pressure molding was conductedin accordance with a conventional method. In this molding, a pressure ofabout 40 kg/cm² was applied to the prepreg layered structure, and, inthis condition, a pressure application started from 110 degrees C. Then,the prepreg layered structure was maintained at 160 degrees C. for anhour. Subsequently, the prepreg layered structure was subjected to aheat treatment at 260 degrees C. for 16 hours using a dryer, to form apreform.

The preform was baked through a heat treatment in a stream of nitrogengas at a temperature rise rate of about 10 degrees C./hour up to 1000degrees C. Thereby, a C/C composite was obtained. An obtained materialhad a bulk density of 1.51 g/cm³ and a bending strength of 110 MPa.

EXAMPLE 22

Boron carbide having an average particle diameter of 3 μm was added intoethyl alcohol, and well stirred and dispersed. An amount of the boroncarbide was 32 parts by volume based on 100 parts by volume of carbonfibers. The boron carbide powder was in advance sufficiently mixed andcrushed with shearing force applied thereon by using a super mixer. Thesuper mixer performed mixing for three minutes at a blade rotationalspeed of 1500 rpm. The ethyl alcohol was 2.5 times the boron carbide byweight. A resol phenolic resin as a liquid synthetic resin was addedthereto at a weight ratio of 0.8 with respect to carbon fibers, and thenwell stirred and dispersed to be made into slurry, so that a matrixprecursor was prepared. The slurry was uniformly applied to fine-wovenspun-yarn cloths of 2D cloths (which are PAN-based ones having a fiberdiameter of about 10 μm) using a doctor blade. At this time, in order toinfiltrate the slurry into the cloths, this process involved aningenuity of applying in advance a part of the slurry to the cloths in arubbing manner and then applying the remaining slurry to surfaces of thecloths. Here, a viscosity of the slurry at a working temperature was 9mPa-s. The cloths having the slurry applied thereto were put in layersand air-dried. The cloth had a size of 80 mm×80 mm, and fifteen clothswere put in layers to form a prepreg layered structure having athickness of about 5 mm. Next, a thermal pressure molding was conductedin accordance with a conventional method. In this molding, a pressure ofabout 40 kg/cm² was applied to the prepreg layered structure, and, inthis condition, a pressure application started from 110 degrees C. Then,the prepreg layered structure was maintained at 160 degrees C. for anhour. Subsequently, the prepreg layered structure was subjected to aheat treatment at 260 degrees C. for 16 hours using a dryer, to form apreform.

The preform was baked through a heat treatment in a stream of nitrogengas at a temperature rise rate of about 10 degrees C./hour up to 1000degrees C. Thereby, a C/C composite was obtained. An obtained materialhad a bulk density of 1.48 g/cm³ and a bending strength of 45 MPa.

COMPARATIVE EXAMPLE 1

A C/C composite was obtained through the same procedure as in Example 2,except that a volume fraction of boron carbide powder in a matrix was 50volume %.

COMPARATIVE EXAMPLE 2

A C/C composite was obtained through the same procedure as in Example 1,except that a volume fraction of boron carbide powder in a matrix was 50volume %.

COMPARATIVE EXAMPLE 3

A C/C composite was obtained through the same procedure as in Example 1,except that boron carbide powder had an average particle diameter of 10μm and that a volume fraction of the boron carbide powder in a matrixwas 52 volume %.

COMPARATIVE EXAMPLE 4

A C/C composite was obtained through the same procedure as in Example16, except that both boron carbide powder and silicon carbide powder ina matrix had an average particle diameter of 10 μm.

COMPARATIVE EXAMPLE 5

A C/C composite was obtained through the same procedure as in Example 1,except that a matrix contained no ceramic powder.

COMPARATIVE EXAMPLE 6

A C/C composite was obtained through the same procedure as in Example13, except that a matrix contained no ceramic powder. In ComparativeExamples 5 and 6, after applying a resin to cloths, a molding and a heattreatment were performed in accordance with the procedure of Examples 1and 13.

COMPARATIVE EXAMPLE 7

A C/C composite was obtained through the same procedure as in Example20, except that added was boron carbide powder having an averageparticle diameter of 3 μm in an amount of 29 parts by volume based on100 parts by volume of carbon fibers. Here, a viscosity of the slurry ata working temperature was 10 mPa-s. An obtained material had a bulkdensity of 1.32 g/cm³ and a bending strength of 32 MPa.

FIGS. 1(a, 1(b), and 1(c) are tables showing Examples and ComparativeExamples collectively. FIGS. 1(a), 1(b), and 1(c) show types and volumefractions of carbon fibers, types of carbon precursor, weight ratios ofcarbon precursors to carbon fibers, types of ceramic powder, weightratios and volume fractions of ceramic powders to carbon fibers, andvolume fractions of ceramic powders in a matrix after a final heattreatment. FIGS. 1(a) and 1(b) show mix proportions of the materials inExamples and Comparative Examples. The matrix comprises a carbonizedcarbon precursor and ceramic powder. As characteristics of the obtainedC/C composites, shown are bulk densities, bending strengths, oxidationlosses in the atmosphere (FIG. 1(c)).

All the materials used in these Examples were commercially availableones. As the carbon fibers, used were (1) fine-woven spun-yarn clothsthat are two-dimensional cloths (which are PAN-based ones having a fiberdiameter of about 10 μm); (2) plain-woven cloths of continuous threadsof long carbon fibers that are two-dimensional cloths (which arePAN-based ones having a tensile strength of 3500 MPa, a tensile modulusof 230 GPa, a fiber diameter of about 7 μm, and a filament number of6000 (as called “6K”)); (3) one-dimensional sheets of continuous threadsof long carbon fibers (which are pitch-based ones having a tensilestrength of 3600 MPa, a tensile modulus of 650 GPa, a fiber diameter of10 μm, and a sheet thickness of 0.29 mm); and (4) short fibers (whichare PAN-based ones having fiber lengths of 30 μm and 1000 μm). As theceramic powder, used were (1) boron carbide powder (having averageparticle diameters of 1 μm, 3 μm, and 10 μm); and (2) silicon carbidepowder (having average particle diameters of 1 μm, 3 μm, and 10 μm). Theceramic powder was used in a commercially available form, oralternatively used after classification. The particle diameters wereexamined with a laser-diffraction-type particle-distribution measurementdevice (SALD-2000A manufactured by Shimadzu Corporation). As the carbonprecursor in the matrix, used were (1) a resol phenolic resin; (2) apowdery phenolic resin; (3) mesocarbon microbeads; and (4) carbon powder(having an average particle diameter of 5 μm and 10 weight % of volatilematter, obtained by hot-kneading and then pulverizing pitch andartificial graphite that has an average particle diameter of 1 μm).

In order to determine a volume fraction of the ceramic in the matrix,examined beforehand were carbonization yield of the carbon precursor anda true density of a carbon compound resulting from the carbon precursor.At 1000 degrees C., the mesocarbon microbeads had a carbonization yieldof 90% and a true density of 1.9 g/cm³, and the carbon powder had acarbonization yield of 90% and a true density of 2.0 g/cm³. At 1000degrees C., the resol phenolic resin had a carbonization yield of 50%and a true density of 1.5 g/cm³, and the powdery phenolic resin had acarbonization yield of 70% and a true density of 1.5 g/cm³. A truedensity of the boron carbide powder was 2.5 g/cm³, and a true density ofthe silicon carbide powder was 3.1 g/cm³.

A bending strength specimen used was a rectangular solid of 4 mm×8 mm×75mm. In case of two-dimensional cloths, a surface direction of the clothswas defined as a longitudinal direction. In case of one-dimensionalsheets, a fiber direction was defined as a longitudinal direction. Adirection having 4 mm-length was a thickness direction of the cloths orsheets. A bending test was performed in a three-point bending methodusing an Instron testing machine under conditions of a span of 60 mm, acrosshead speed of 0.5 mm/min, and in the room temperature. It has beenconfirmed that any oxidation resistant C/C composite of the presentExamples incurs no brittle fracture, exhibits the same crossheaddisplacement as usually exhibited by a commercially available 2D-C/Ccomposite, and has the same toughness as that of a basic C/C composite.

The boron carbide powder having the smaller particle diameter resultedin the higher bending strength (compare, e.g., Example 1 with Example2). A use of the powdery phenolic resin in combination resulted in animprovement in the bending strength (see, e.g., Examples 6, 15, and 16).This is presumably because the compositing of fine particles developsits effects well.

Oxidation loss in the atmosphere was tested at 800 degrees C., 900degrees C., 1000 degrees C., and 1200 degrees C. using a commerciallyavailable electric furnace. A specimen was taken out of the electricfurnace every hour to be weighed, so that a rate of weight change wasdetermined. The C/C composites that contain, among the ceramic powders,only the boron carbide were weighed at 900 degrees C. at the most,because they exhibited monotonous weight loss at 900 degrees C. The C/Ccomposites of Comparative Examples 5 and 6 containing no ceramic powderwere weighed at 900 degrees C. at the most, because they exhibitedconsiderable weight loss at 900 degrees C. in comparison with thosecontaining ceramic powder.

FIGS. 2 to 6 show variations in weight loss of some Examples andComparative Examples due to oxidation loss at the respectivetemperatures as plotted against lapse of time.

As shown in FIG. 2, at 800 degrees C., any Example exhibited a littleweight increase at an initial stage of the oxidation loss test (weightincrease is also denoted by zero, “0”, in FIG. 1). There was observed notendency of weight loss, and oxidation loss was substantiallysuppressed. Weight increase occurs because ceramic powder, particularlyexisting on a surface of a material, is oxidized to become an oxidecompound. When there is observed weight increase only, it can beconsidered that substantially no oxidation loss is arising. When aninitial stage of the oxidation loss test sees weight loss followed by nofurther weight loss observed, shown is that an oxidation-resistantcoating completely covers a surface of a material to thereby suppresssubsequent oxidation loss thereafter. Monotonous weight loss over timeshows either that an oxidation-resistant coating is not formed uniformlyover an entire surface of a material or that a ceramic oxide isevaporating. In Comparative Examples 1 to 4, monotonous weight loss overtime was caused, though inconsiderably, even at 800 degrees C. Thisshows that, when a volume fraction of ceramic powder in a matrix is 50volume % or when an average particle diameter of ceramic powder is aslarge as 10 μm, an oxidation-resistant coating fails to be formed welluniformly over an entire surface of a material.

As shown in FIG. 3, among the materials of the present invention, thematerials containing only the boron carbide powder exhibited, thoughinconsiderably, monotonous weight loss over time at 900 degrees C. Thisis presumably because of evaporation of boron oxide. Even at 900 degreesC., the materials of the present invention exhibited less weight lossnot only than the materials containing no boron carbide powder(Comparative Examples 5 and 6) but also than the materials inComparative Examples 1 to 4. The materials containing both boron carbidepowder and silicon carbide powder exhibited no weight loss, andoxidation loss thereof was almost completely suppressed even at 900degrees C. In Comparative Examples 5 and 6, at 900 degrees C., almostall the specimens disappeared in five hours due to oxidation.

As shown in FIG. 4, at 1000 degrees C., the materials of Examples 10 and16 that contain the boron carbide powder and the silicon carbide powderexhibited no weight loss, and oxidation loss thereof were almostcompletely suppressed even at 1000 degrees C. In Comparative Example 4,weight loss arose at an initial stage.

As shown in FIG. 5, at 1200 degrees C., the materials of Examples 10 and16 that contain the boron carbide powder and the silicon carbide powderexhibited weight loss at initial stages of oxidation loss, and,thereafter, hardly exhibited further weight loss. Comparative Example 4exhibited monotonous weight loss over time.

FIG. 6 shows variations in weight loss of Examples 19 to 22 andComparative Example 7 due to oxidation loss at 800 degrees C. as plottedagainst lapse of time. As shown in FIG. 6, at 800 degrees C., anyExample exhibited a little weight increase at an initial stage ofoxidation loss. No weight loss was exhibited, and oxidation loss wassubstantially suppressed. It can be seen from Comparative Example 7that, when the amount of boron carbide was less than 32 volume % basedon the volume of the carbon fibers, weight loss due to oxidation lossoccurred little by little.

As described above, according to the oxidation resistant C/C compositesof the present invention which contain, among the ceramic powders, onlythe boron carbide powder, weight loss due to oxidation was completelysuppressed at 800 degrees C. At 900 degrees C. as well, weight loss dueto oxidation could extremely be reduced. According to the oxidationresistant C/C composites of the present invention which contain, amongthe ceramic powders, both the boron carbide powder and the siliconcarbide powder, weight loss due to oxidation was completely suppressedup to 1000 degrees C. At 1200 degrees C., although weight loss of notmore than 10% occurred at the initial stage, further weight loss wassuppressed thereafter. Accordingly, they can be adopted as anoxidation-resistant material up to 1200 degrees C.

Industrial Applicability

There can be produced through a simple process an oxidation resistantC/C composite that suffers from substantially no oxidation loss at 800degrees C. in the atmosphere and has excellent mechanicalcharacteristics such as high toughness etc.

1. An oxidation resistant carbon fiber reinforced carbon composite material comprising a matrix and 20 volume % or more of carbon fibers, wherein: the matrix contains ceramic powder that includes boron carbide powder having an average particle diameter of 5 μm or less; and an amount of the ceramic powder is 32 volume % or more based on volume of the carbon fibers.
 2. The oxidation resistant carbon fiber reinforced carbon composite material according to claim 1, wherein the carbon fibers comprise any one of: a fine-woven spun-yarn cloth; a two-dimensional-woven cloth of continuous threads of long carbon fibers; a one-dimensional long carbon fiber sheet; short carbon fibers and a fine-woven spun-yarn cloth; short carbon fibers and a two-dimensional-woven cloth of continuous threads of long carbon fibers; and short carbon fibers and a one-dimensional long carbon fiber sheet.
 3. The oxidation resistant carbon fiber reinforced carbon composite material according to claim 1, wherein the ceramic powder consists of boron carbide powder.
 4. The oxidation resistant carbon fiber reinforced carbon composite material according to claim 1, wherein the ceramic powder comprises boron carbide powder and silicon carbide powder.
 5. The oxidation resistant carbon fiber reinforced carbon composite material according to claim 1, wherein the matrix includes a carbon compound resulting from liquid synthetic resin or from liquid synthetic resin and powdery synthetic resin.
 6. The oxidation resistant carbon fiber reinforced carbon composite material according to claim 5, wherein the matrix includes mesocarbon microbeads and/or carbon powder having 5 to 15 weight % of residual volatile matter.
 7. The oxidation resistant carbon fiber reinforced carbon composite material according to claim 1, produced by uniformly applying a slurried precursor of the matrix to cloth-form or sheet-form carbon fibers, which are subsequently put in layers, baked, and further impregnated with pitch or synthetic resin to be carbonized.
 8. A process for producing an oxidation resistant carbon fiber reinforced carbon composite material comprising a matrix and 20 volume % or more of carbon fibers, wherein a slurried matrix precursor formed by mixing liquid synthetic resin with ceramic powder that includes, based on volume of the carbon fibers, 32 volume % or more of boron carbide powder having an average particle diameter of 5 μm or less are uniformly applied to the cloth-form or sheet-form carbon fibers, which are subsequently put in layers and baked.
 9. The process for producing an oxidation resistant carbon fiber reinforced carbon composite material according to claim 8, wherein the matrix precursor is uniformly infiltrated into cloth-form or sheet-form carbon fibers and, further, uniformly applied to surfaces of the carbon fibers, which are subsequently put in layers and baked.
 10. The process for producing an oxidation resistant carbon fiber reinforced carbon composite material according to claim 8, wherein the matrix precursor is uniformly applied to cloth-form or sheet-form carbon fibers, which are subsequently put in layers, baked, and further impregnated with pitch or synthetic resin to be carbonized.
 11. The process for producing an oxidation resistant carbon fiber reinforced carbon composite material according to claim 8, wherein the matrix precursor has a viscosity of 5 to 40 mPa-s.
 12. The process for producing an oxidation resistant carbon fiber reinforced carbon composite material according to claim 8, wherein the matrix precursor contains any one or more among mesocarbon microbeads, carbon powder including 5 to 15 weight % of residual volatile matter, and short carbon fibers.
 13. A process for producing an oxidation resistant carbon fiber reinforced carbon composite material comprising a matrix and 20 volume % or more of carbon fibers, wherein ceramic powder and any one or more among synthetic resin powder, mesocarbon microbeads, carbon powder including 5 to 15 weight % of residual volatile matter, and short carbon fibers are integrally made into solid fine particles to obtain powder, and a slurried matrix precursor mixed with the obtained powder is uniformly applied to surfaces of cloth-form or sheet-form carbon fibers, which are subsequently put in layers and baked.
 14. The process for producing an oxidation resistant carbon fiber reinforced carbon composite material according to claim 13, the material comprising a matrix and 20 volume % or more of carbon fibers, wherein ceramic powder and any one or more among synthetic resin powder, mesocarbon microbeads, carbon powder including 5 to 15 weight % of residual volatile matter, and short carbon fibers are integrally made into solid fine particles to obtain powder, and a slurried matrix precursor mixed with the obtained powder is uniformly applied to surfaces of cloth-form or sheet-form carbon fibers, which are subsequently put in layers, baked, and further impregnated with pitch or synthetic resin to be carbonized.
 15. The process for producing an oxidation resistant carbon fiber reinforced carbon composite material according to claim 13, wherein the matrix precursor mixed with the obtained powder has a viscosity of 5 to 40 mPa-s. 